Air leak problems

[Jonny P]

Morning all,

Having recently bought my X Type it started showing (which I now realise are very common) symptoms of an air leak in the vacuum system somewhere. I have invested in a code reader which is showing both codes P0171 and P0174 which also points to this. Having replaced the two O rings on the sensors connected to the intake manifold and also changing the breather pipe between the air intake manifold and the engine for the updated ribbed version, the problem persisted. 

Rather than carrying on replacing everything it could be, I decided to try to find out where the leak is. I've sprayed carb cleaner around all points where there could be a leak but there's been no increase in engine revs, as you'd expect. Next on the list of parts to replace is the brake booster pipe/valve.

Having read about this problem in great detail all over the web, I have to say my problem differs in as much as the warning light and "cruise not available" warning only comes on at motorway speeds, usually as I hit 70mph (I do a 70 mile round trip most days) at which point I pull over, wait for the revs to drop to idle speed, the warning clears and the light goes off and I can then drive on with no problems or warning for the rest of the journey. The main check engine symbol stays lit.

It's this that's stopping me just replacing everything, surely if there's a leak it wouldn't "reset" just because I pull over and let the revs drop? Or would it? Any help would be greatly appreciated.

Thanks

Jonn P